FAIR VALUE MEASUREMENTS - Estimated Fair Values for All Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Financial liabilities:
Other secured financing	$ 4,108,547		$ 3,837,528
Carrying Value | Level 2
Financial assets:
Corporate debt	1,528,874		1,011,275
Financial liabilities:
Repurchase agreements	79,073,026		77,696,343
Other secured financing	4,108,547		3,837,528
Carrying Value | Level 3
Financial assets:
Commercial real estate debt and preferred equity, held for investment	1,435,865		1,029,327
Commercial loans held for sale, net		$ 42,325	0
Financial liabilities:
Mortgage payable	511,588		309,686
Fair Value | Level 2
Financial assets:
Corporate debt	1,531,546		1,014,139
Financial liabilities:
Repurchase agreements	79,073,026		77,697,828
Other secured financing	4,108,801		3,837,595
Fair Value | Level 3
Financial assets:
Commercial real estate debt and preferred equity, held for investment	1,446,062		1,035,095
Commercial loans held for sale, net		$ 43,055	0
Financial liabilities:
Mortgage payable	$ 494,690		$ 310,218